There are two ways:
– via travel agents:
To look for the promotion fares offered by airlines or local travel agents, e.g. travel fairs. However, these so-called promotional ares are sometimes very hard to obtain during peak periods (e.g. school holidays) as the seats for those fares are limited. For people travelling outside the park periods or are flexible with their travel dates, this is one of the best options.
– Online fares (early-bird or promotional)
Go directly to airline websites and purchase online. But watch out for the restrictions for different fares such as charges for change/cancellation/refund.